The present invention is in the field of optical waveguides, and particularly relates to the production of optical waveguides or optical waveguide blanks or preforms directly from molten glass.
Although present commercial high-bandwidth optical waveguide filaments are produced by processes involving the chemical vapor deposition of glass waveguide components to form pure waveguide blanks or preforms, it has also been proposed to produce such products by direct drawing from molten glass. Hence, U.S. Pat. No. 3,726,656 describes an updrawing process for producing glass-clad glass rod which can be used as a blank or preform for redrawing optical waveguide filaments, while U.S. Pat. Nos. 3,941,474, 3,791,806 and 4,032,313 describe the manufacture of glass-clad glass waveguide filaments by direct down-drawing from nested multiple crucibles or reservoirs of molten glass.
The direct drawing of optical waveguides from molten glass produced by a chemical vapor deposition process is described in U.S. Pat. No. 4,065,280, while published German Application No. 2,614,631 describes the production of multilayer optical waveguides by an updraw process.
Molten glass drawing methods have been proposed for the manufacture of both step-index and graded-index optical waveguides. In the case of step-index waveguides, only two crucibles or other reservoirs of molten glass are needed. These are concentrically positioned so that a relatively low refractive index glass for the waveguide cladding layer is drawn from an outer reservoir and formed around a relatively high refractive index glass for the waveguide core which is simultaneously drawn from an inner reservoir. In the case of a graded-index waveguide, multiple crucibles or reservoirs of molten glass are used, with the glass in the outer reservoir forming the optical waveguide cladding layer and the glasses in the inner reservoirs forming a core group which ultimately becomes the core member of the optical waveguide. Glasses which increase step-wise in refractive index from the periphery of the core group to the center thereof are used to approximate the parabolic core index profile which is desirable for high waveguide bandwidth.
The refractive indices of the glasses forming the core and cladding layers in the multiple crucible assembly are controlled through the use of additives, termed dopants, which typically act to raise the refractive index of a glass in which they are dissolved to an extent proportional to their concentration therein (although dopants which reduce refractive index are also known). At the elevated temperatures employed for the direct drawing of waveguides or waveguide blanks from molten glass, these refractive-index-controlling dopants tend to diffuse or migrate from one glass layer to another. This has not been considered to be objectionable since the effect is to smooth the refractive index profile in the core group and thus to better approximate the parabolic index distribution necessary for high bandwidth.
However, one disadvantageous effect of dopant migration during the manufacture of optical waveguide filaments or blanks or molten glass is the migration which occurs between the outermost core members and the cladding layer. This core/cladding diffusion of dopants results in diffusion "tails", which are localized increases in the refractive index of the cladding near the core/cladding boundary. These tails reduce the definition of the boundary and undesirably decrease the bandwidth of the optical waveguide.
It is a principal object of the present invention to provide a method for the direct drawing of optical wave-guides wherein the problem of core/cladding diffusion is avoided.
Other objects and advantages of the invention will become apparent from the following description thereof.